Note: Descriptions are shown in the official language in which they were submitted.
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AIR DEMAND FEEDBACK CONTROL SYSTEMS AND METHODS FOR SULFUR
RECOVERY UNITS
This application claims priority to our copending U.S. provisional patent
application
with the serial number 60/945495, which was filed June 21, 2007.
Field of The Invention
The field of the invention is control systems for Claus plant sulfur recovery
units, and
especially control systems for Claus plants with multiple thermal stages.
Background of the Invention
Sulfur compounds (and especially hydrogen sulfide) are typically removed from
a
waste gas prior to venting into the atmosphere using one or more Claus plant
sulfur recovery
units that include a thermal stage followed in series by one or more catalytic
stages. While
sulfur recovery using a Claus plant is conceptually simple and well known in
the art, effective
operation of a Claus plant is not trivial due to numerous variable parameters.
Among other
factors, the chemical composition (e.g., content and relative proportions of
hydrogen sulfide,
carbon dioxide, and water) of the feed stream into the Claus plant may change
considerably
dependent on the type of facility and processes used upstream of the Claus
plant. Therefore,
based on the specific stoichiometric requirements of the Claus reaction,
stringent control of
oxygen quantities for the thermal stage is critical for effective operation of
a Claus plant.
In most currently known configurations with a single thermal stage, constant
chemical
composition of the waste gas feed is assumed for control of the amount of
oxygen needed in
the thermal stage. To accommodate small variations in hydrogen sulfide
concentration in the
feed gases, and to account for inaccuracies in the thermal stage control
instrumentation, it is
common practice to install feedback control instrumentation in such plants to
make fine
adjustments to the thermal stage air demand logic. The feedback logic in such
systems
typically involves gas analysis of the molar ratio of hydrogen sulfide to
sulfur dioxide in the
tail gas leaving the final catalytic stage. A control signal from a tail gas
analyzer is then used
to make small changes to the quantity of air or other oxygen-containing gas
that is delivered
to the thermal stage to achieve the desired ratio of hydrogen sulfide to
sulfur dioxide in the
effluent stream. A typical example for such configuration is described in U.S.
Pat. No.
4,100,266 where flow of an oxygen-containing gas is regulated using a
controller that
operated on the basis of measured oxygen concentration in the oxygen-
containing gas and
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measured concentration of various components in the tail gas and vented gas
stream.
Similarly, RE028864 describes a system in which a control signal to regulate
flow of oxygen
or oxygen containing gas is generated from (a) measured concentrations of
hydrogen sulfide
and oxygen at the inlet of the thermal stage and a corrective value, and (b)
measured
concentrations of components in the tail gas.
In further known configurations (e.g., WO 2006/005155, or U.S. Pat. No.
3,026,184),
process control is achieved using measurements downstream of both the thermal
stage and
the catalytic stage to form a combined control signal that is then used to
directly regulate the
flow of the oxygen-containing gas to the thermal stage. Combined control
signals allow for
increased fine-tuning of oxygen flow based on two process points, however,
will typically not
allow differentiation between imbalances at the two process points.
Alternatively, temperature control of the thermal stage may be employed to
optimize
the overall performance of a Claus plant as described in U.S. Pat. No.
4,543,245, and in yet
another known approach, oxygen feed to the thermal stage can be controlled by
calibrating a
hydrocarbon-representative response signal (rather than a hydrogen sulfide
representative
response signal) that is responsive to the ratio of hydrogen sulfide/sulfur
dioxide in the Claus
plant tail gas as described in U.S. Pat. No. 4,836,999.
While such known tail gas control circuits tend to operate satisfactorily
under many
circumstances, various difficulties remain, especially in relatively large
Claus plants that need
to process very large quantities of sour feeds. Such plants often include
several thermal
stages operating in parallel followed by one or more catalytic stages
operating in series.
Unfortunately, such known configurations with parallel thermal stages present
problems with
feedback control from the tail gas analyzer (typically measuring ratio of
hydrogen sulfide to
sulfur dioxide). For example, the desired tail gas ratio may not be achieved
where one of the
thermal stages operates with too much air or oxygen while the other thermal
stage(s)
operate(s) with the correct amount or too little air. Since in such plants the
tail gas analyzer
is positioned downstream of the common catalytic stage, the downstream
analyzer is
insensitive to differences between the independently operating thermal stages.
As such, the
analyzer's feedback control signal will take the correct action for one of the
thermal stages,
but an incorrect action for the other(s), potentially intensifying the
problem. Thus, control of
the Claus plant may continually swing from tail gas hydrogen sulfide to sulfur
dioxide ratios
that are too high, to ratios that are too low.
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To circumvent at least some of the problems associated with plants having
multiple
parallel thermal stages, a Claus plant configuration can be implemented in
which oxygen flow
control to the additional thermal stage is achieved by measuring the flow rate
of combustible gas
into the additional thermal stage and the ratio of hydrogen sulfide to sulfur
dioxide in the sulfur
depleted gas stream from the additional thermal stage as described in U.S.
Pat. No. 6,287,535.
While such configurations and methods advantageously allow for significantly
increased
throughput of combustible acid gas, several problems nevertheless remain. Once
more, any
deviation of a desired ratio between hydrogen sulfide to sulfur dioxide in the
tail gas can not be
traced back to a particular thermal stage that produced or precipitated the
deviation.
Therefore, while numerous methods of operational control for Claus plants are
known in
the art, all or almost all of them suffer from one or more disadvantages.
Thus, there is still a need
to provide improved configurations and methods for control in Claus plants.
Summary of the Invention
The present invention is directed to configurations and methods of control of
a Claus
plant having multiple parallel thermal stages in which measurement of effluent
composition from
the thermal stages and from the catalytic stage(s) is used to produce control
signals that
independently allow changing of one or more operational parameters of one or
more thermal
stages.
In one aspect of the present invention, there is provided a method of
controlling operation
of a Claus plant, comprising: (a) monitoring chemical composition of first and
second thermal
stage effluents, wherein the first and second thermal stages operate in
parallel; (b) monitoring
chemical composition of a catalytic stage effluent that is coupled to first
and second thermal
stage to receive the first and second thermal stage effluents; and (c)
independently adjusting an
operational parameter in at least one of the first and second thermal stages
based on results
obtained from steps (a) and (b).
In another aspect of the present invetnion, there is provided a Claus plant
control system
that comprises: a first effluent analyzer operationally coupled to a first
thermal stage of a Claus
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plant, and a second effluent analyzer operationally coupled to a second
thermal stage of the
Claus plant, wherein first and second thermal stages are configured to operate
in parallel; a first
controller operationally coupled to the first thermal stage, wherein the first
controller is
configured to control a first operational parameter of the first thermal
stage; a second controller
operationally coupled to the second thermal stage, wherein the second
controller is configured to
control a second operational parameter of the second thermal stage; a third
effluent analyzer
operationally coupled to a catalytic stage of a Claus plant, wherein the
catalytic stage of the
Claus plant is configured to receive combined effluent from the first and
second thermal stage;
and a control unit that is coupled to the first, second, and third effluent
analyzers and
programmed to allow independent adjustment of the first and second operational
parameters of
the first and second controllers.
In one aspect of the inventive subject matter, a method of controlling
operation of a
Claus plant comprises a step of monitoring chemical composition of first and
second thermal
stage effluents, wherein the first and second thermal stages operate in
parallel, and a further step
of monitoring the chemical composition of a catalytic stage effluent where the
catalytic stage is
coupled to first and second thermal stage to receive the first and second
thermal stage effluents.
In yet another step, an operational parameter is independently adjusted in at
least one of the first
and second thermal stages based on results obtained from the measurements of
the thermal and
catalytic stage effluents.
Most preferably, the step of monitoring of the chemical composition of the
thermal and/or
catalytic stage effluent comprises measuring of hydrogen sulfide and/or sulfur
dioxide
concentrations, and most typically includes measuring of a ratio of hydrogen
sulfide to sulfur
dioxide. Based on the measured concentrations, it is generally preferred that
a first and/or a
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second control signal are calculated, and that the first and/or second control
signals are used
to adjust one or more operational parameters of at least one of the first and
second thermal
stages. Additionally, it is contemplated that the temperature in at least one
of the first and
second thermal stages is measured. Consequently, suitable operational
parameters include the
flow rate of air, the flow rate of an oxygen-containing gas, the flow rate of
sour gas into the
first and/or second thermal stages, and/or the temperature in the first and/or
second thermal
stages.
Therefore, and viewed from a different perspective, a Claus plant control
system has a
first effluent analyzer that is operationally coupled to a first thermal stage
of a Claus plant,
and a second effluent analyzer that is operationally coupled to a second
thermal stage of the
Claus plant, wherein first and second thermal stages are configured to operate
in parallel. In
such plants, a first controller is operationally coupled to the first thermal
stage and configured
to control a first operational parameter of the first thermal stage, and a
second controller is
operationally coupled to the second thermal stage and configured to control a
second
operational parameter of the second thermal stage. A third effluent analyzer
is operationally
coupled to a catalytic stage of a Claus plant, wherein the catalytic stage is
configured to
receive the combined effluent from the first and second thermal stage. A
control unit is
coupled to the first, second, and third effluent analyzers and programmed or
otherwise
configured to allow independent adjustment of the first and second operational
parameters of
the first and second controllers.
Most typically, the first, second, and/or third effluent analyzers are
configured to
measure a ratio of hydrogen sulfide to sulfur dioxide, and first and second
temperature
analyzers may be coupled to the first and second controllers. In especially
contemplated
plants, the control unit is programmed to provide first and second control
signals to the first
and second controllers, respectively, to thereby independently adjust set
points for a ratio of
hydrogen sulfide to sulfur dioxide for the first and second thermal stages.
Alternatively, or
additionally, the control unit may also be programmed to provide first and
second control
signals to the first and second controllers, respectively, to thereby
independently adjust
operating temperature of the first and second thermal stages. Where desirable,
the control
unit may be integrated with the first controller.
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Various objects, features, aspects and advantages of the present invention
will become
more apparent from the following detailed description of preferred embodiments
of the
invention, along with the accompanying drawing.
Brief Description of the Drawing
Figure 1 is a schematic illustration of an exemplary Claus plant control
configuration
according to the inventive subject matter.
Detailed Description
The inventors have discovered that control systems for a Claus plant having
multiple
parallel thermal stages upstream of a series of common catalytic stages can be
implemented
such that operation of each of the thermal stages can be individually
controlled to provide a
an effluent with a desired hydrogen sulfide to sulfur dioxide ratio.
Therefore, in particularly
contemplated configurations and methods, a gas analyzer is provided to the
outlet of each
thermal stage and configured to analyze the hydrogen sulfide to sulfur dioxide
ratio in the gas
stream exiting the respective stages. Based on a predetermined and desired
ratio of hydrogen
sulfide to sulfur dioxide in the common downstream catalytic stage tail gas,
and based on the
actual ratio of hydrogen sulfide to sulfur dioxide in the individual thermal
stages of the Claus
plant, a processing unit will calculate the required corresponding corrected
ratios for each of
the outlets of the upstream thermal stages. Thus, a calculated feedback
control signal from
the tail gas analyzer resets the hydrogen sulfide to sulfur dioxide ratio set-
point at each of the
thermal stages' gas analyzer controllers. Consequently, each gas analyzer
controller of the
respective thermal stage will be used to make the small additions or deletions
to the quantity
of air (or other oxygen-containing gas) that is delivered to that thermal
stage in order to attain
the desired ratio of hydrogen sulfide to sulfur dioxide in that stage's
effluent stream.
In one exemplary aspect of the inventive subject matter as schematically
illustrated in
Figure 1, a Claus plant 100 includes two thermal stages 110A and 110B
operating in parallel,
and two catalytic stages 120A and 120B that receive the combined effluents of
the upstream
thermal stages 110A and 110B and operate serially. Thermal stage effluents
112A and 112B
are analyzed by respective sensors 130A and 130B and associated
analyzers/controllers 132A
and 132B. Most preferably, the analyzer signals from analyzers/controllers
132A and 132B
are generated as a function of the signals from the corresponding sensors and
transmitted to
the analyzer/controller 152. Catalytic stage effluent 122 is analyzed by
respective sensor 150
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and corresponding analyzer/controller 152. The analyzer/controller 152 is then
programmed
to individually re-adjust the initial set-points of analyzers/controllers 132A
and 132B based
on the analyzer signals from analyzers/controllers 132A and 132B and the
analyzer signals
from analyzer/controller 152 to so individually and independently regulate the
composition of
effluents 112A and 112B and thus catalytic stage effluent 122. Therefore, it
should
appreciated that there may be unidirectional or bidirectional flow of
information between the
analyzer/controller 152 and analyzers/controllers 132.
In alternative aspects of the inventive subject matter it should be
appreciated that the
individual control of the various operational parameters may be implemented in
manners
other than those described above for Figure 1. For example, it is contemplated
that a separate
control unit may receive signals from all sensors and/or analyzers, wherein
the so transmitted
signals correspond to the measured value of one or more operational parameters
of the
thermal stage(s) and/or catalytic stage. Such central control unit may then be
used to control
the operational parameters of all thermal and catalytic stages. On the other
hand, and where
desirable, it is contemplated that each of the analyzers/controllers may also
include at least
part of the control unit and so obviate a central control unit. Such
analyzers/controllers will
then be configured to allow coupling to at least two other
analyzers/controllers to allow uni-
and more preferably bidirectional communication with each other.
With respect to the type of sensors, and analyzers/controllers, it should be
appreciated
that all known sensors and analyzers/controllers are deemed suitable for use
herein. However,
it is especially preferred that the sensors provide real-time or near real-
time (e.g., lag time
less than 10 minutes, more preferably less than 5 minutes) compositional
information for at
least one component in the effluent. Most typically, the sensor therefore
comprises an optical
component (e.g., spectroscopic sensor) and in less preferred aspects (e.g.,
where fluctuations
are relatively slow) a chromatographic component. It should still further be
appreciated that
the sensor may be suitable to measure a single component (e.g., hydrogen
sulfide) or multiple
components (e.g., hydrogen sulfide and sulfur dioxide), or surrogate markers
thereof. On the
other hand, multiple sensors are also contemplated that may be used to measure
multiple
components.
It is generally preferred that the sensor for the thermal stage is positioned
upstream of
the entry point of the combined effluents to the catalytic stage. For example,
where the
thermal stage has a downstream sulfur condenser, it is contemplated that the
sensor is6
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positioned at or near the condenser outlet. On the other hand, where no sulfur
condenser is
present, the sensor is typically positioned in close proximity of the thermal
stage outlet. In
still further contemplated aspects of the inventive subject matter, it is
contemplated that an
additional sensor may be positioned such that the additional sensor senses the
concentration
of at least one component in a combined thermal stage effluent stream, which
may be
especially advantageous where three or MOM thermal stage effluents arc
combined. The
additional sensor may then be coupled to the control unit to provide further
information on
potential stoichiometric imbalances. Similarly, it is contemplated that
multiple sensors in a
train of catalytic stages may be provided where two, three, or even more
catalytic stages arc
to employed.
With respect to suitable analyzers and/or controllers it is contemplated that
all known
analyzers and process controllers for adjusting air and/or oxygen feed, sour
gas feed. and/or
temperature in the thermal stage arc suitable for use herein. Therefore, such
analyzers and/or
controllers may be integrated into a single device or be provided as separate
components. As
pointed out above, the controller may also include at least part of the
control unit that
modifies the controller of the thermal stage.
Consequently, it should be appreciated that precise control of the quantity of
air or
other oxygen-containing gas delivered to the thermal stage can now be
achieved, which is
critical to proper operation of a Claus plant. In contrast, currently known
tail gas feedback
control strategies cannot achieve such regulation where the catalytic stage is
preceded by
multiple parallel thermal stages.
Thus, specific embodiments and applications of control systems for sulfur
recovery
units have been disclosed. It should be apparent, however, to those skilled in
the art that
many more modifications besides those already described are possible without
departing
from the inventive concepts herein. The inventive subject matter, therefore,
is not to be
restricted except in the spirit of the present disclosure. Moreover, in
interpreting the
specification and contemplated claims, all terms should be interpreted in the
broadest
possible manner consistent with the context. In particular, the terms
"comprises" and
"comprising" should be interpreted as referring to elements, components, or
steps in a non-
exclusive manner, indicating that the referenced elements, components, or
steps may be
present, or utilized, or combined with other elements, components, or steps
that are not
expressly referenced.
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